The present invention relates to a control signal reproducing circuit in a magnetic recording and reproducing apparatus having a VISS function, which performs a normal VISS operation by clipping a residual signal having gain incidentally increased according to an increase of tape speed at a certain level, or by suppressing the high band by way of a band adjustment.
Generally, in a video cassette recorder (VCR), during the fast forward (FF)/rewind (REW) mode a capstan motor rotates at N times the normal playback speed according to the equation v=r.times..omega. in which the linear velocity (v) of the tape is varied according to the amount of wound tape. Thus, the frequency of a control pulse reproduced by a control head is a maximum at the end of the tape during the FF mode, and is also at a maximum at the beginning of the tape during the REW mode. Further, since the magnitude of a reproduced control pulse is proportional to the variation of magnetic flux density, the faster the tape's linear velocity is, the greater the magnitude of the control pulse becomes, as well as the higher its frequency. That is, the effective electromagnetic force (emf) is calculated by: emf(.omega.)=j.omega..times..phi.(.omega.) where, .phi.(.phi.)=magnetic flux density.
In general a VISS (video index search system) function is designed to facilitate searching of the contents of a certain recording portion by recording index information on a tape by using the variation of the duty cycle of a control pulse employed to control the phase of the capstan.
However, during a VISS overwriting operation to record the index information on a tape, the previous control track's signal is not completely removed and remains as a residual signal. The residual signal has its gain greatly increased because the tape speed becomes fast during such a VISS operation as introscan or index search, which results in faulty VISS operation. An index search operation is provided to recognize an index-signal-recorded portion and perform a playback operation when searching (by using a FF or REW function) an index signal recorded by varying the duty cycle of the control pulse. An introscan operation performs playback for five seconds in the index-signal-recorded portion, and then re-initiates a FF or REW operation to search the next index-signal-recorded portion.
That is, as shown in FIG. 1, a control pulse reproduced from a control track by an A/C head 10 in a conventional VCR is converted into a square wave by a schmitt trigger 22 via an amplifier 21. Amplifier 21 and schmitt trigger 22 are servo circuits composed of ICs.
During normal playback, a control pulse (FIG. 2A) is recorded to maintain about a 60% duty cycle, and FIG. 2B shows the output of schmitt trigger 22 which processes the control pulse shown in FIG. 2A. During the recording of an index signal for VISS operation, a control pulse (FIG. 2C) is recorded to maintain about a 30% duty cycle by moving the negative-going pulse, and FIG. 2D shows the output of schmitt trigger 22 which processes the control pulse shown in FIG. 2C.
However, as shown in FIG. 2E, during the overwriting of an index signal for VISS operation, the negative-going control pulse previously recorded on the control track is not sufficiently removed and remains as a residual signal; the effects of which are shown in FIG. 2F.
That is, in a VHS mode, the ratio of the main signal level to the residual signal level should be over 20 dB during recording. However, during playback, different from recording, if the playback is a fast speed playback such as a VISS operation (FF or REW mode), the gain level of the residual signal becomes greater than the triggering voltage level of the schmitt trigger (particularly at the beginning and end of a tape). This means that, as seen in FIG. 2E, the residual signal remains, which in turn creates a control pulse having the improper duty cycle for the VISS format because, during fast playback, the gain of the residual signal increases so that its level is greater than the triggering level of the schmitt trigger. This causes faulty operation.